RF amplifiers play a very important role in communication systems. A linear RF amplifier is usually biased for class AB operation to achieve higher power-added efficiency (PAE). In particular, RF amplifiers biased in a class AB mode are often used in high-frequency amplification. Typically, for a conventionally biased class AB amplifier, the average bias supply current increases as the RF input power increases. The continued increase in the average current also results in an increase in the voltage drop of resistive components of the biasing circuit. The increased voltage drop across the resistive components in turn reduces the average voltage drop across the forward biased PN junction of the amplifying transistor and pushes the amplifier into class B and possibly even class C operation which causes the output power of the amplifying transistor to become saturated and the output signal to become increasingly nonlinear. The phenomenon is further accentuated as the input power to the RF amplifier is increased.
In efforts to improve linearity at the higher output power levels when the amplifier is operated in saturation or close to saturation, the bias of the amplifying transistor has to be boosted with additional current during operation near saturation. Prior-art techniques used a cascode current mirror circuit, a modified Wilson current mirror circuit, or a stacked diode circuit with a capacitor to provide bias boosting for the RF amplifying transistor. Disadvantages of the cascode current mirror circuit and the modified Wilson current mirror circuit biasing schemes include the requirement of additional resistors to achieve accurate control of the quiescent current in the amplifying transistor due to a necessary isolation resistor between RF and biasing circuits. Disadvantages of the stacked diode circuit with a capacitor include the greater difficulty of achieving sufficient control of the quiescent current since the stacked diode circuit does not form a current mirror circuit.
Accordingly, it is desirable to provide an RF amplifier that overcomes the disadvantages of current RF amplifier designs. What is needed is a method and apparatus for operating a RF amplifier that provides self-bias boosting for the amplifying transistor.